1. Technical Field
The present invention relates to optical systems and more particularly to an optical system employing an optical fluid having improved performance.
2. Discussion
Optical fluids have a number of important applications in optical systems. In general, optical fluids are used to avoid abrupt shifts in index of refraction, encountered by a light beam, for example, when a light beam passes through an air/glass interface. The optical fluid, if its index matches that of the glass, minimizes this index of refraction change which would degrade optical performance.
One important application for optical fluids is a type of beam splitter which is widely used for selectively passing and reflecting a light beam. Such beam splitters include a thin, flat, parallel sided, transparent plate mounted in a transparent liquid or solid medium at an angle (commonly about 45 degrees), to the axis of the beam of light that is to be transmitted or reflected. Such beam splitters may be polarizing or non-polarizing.
A polarizing beam splitter, but not of the embedded (immersed in optical fluid) type, is described in U.S. Pat. No. 2,403,731 issued to MacNeille. The polarizing beam splitter, such as the MacNeille type polarizing beam splitter will pass light having one polarizing state, such as the "P" state, for example, and reflect light with another polarization state, such as the "S" state. Thus, the polarizing beam splitter selectively passes or transmits a light beam, depending upon whether the polarization vector of the light is one or the other of two mutually orthogonal directions. In the beam splitter described in the MacNeille patent, a plurality of dielectric layers of appropriate indices of refraction and thicknesses are deposited at the interface between the two halves of a glass cube wherein the mating surface extends diagonally between two diagonally opposite edges of the cube.
In an embedded MacNeille polarizing beam splitter, a housing of generally cubic configuration is provided with transparent front, back, entrance and exit windows. This housing is filled with a fluid in which is suspended a prism plate comprising a thin plate with mutually parallel planar sides that extend diagonally across the cube. A plurality of thin dielectric layers, of the type described in the MacNeille patent, may be applied to the thin plate to make this embedded prism a MacNeille polarizing prism. Such embedded prisms exhibit a color defect known as "lateral chromatic aberration", which significantly decrease clarity and resolution of transmitted light and also significantly reduces contrast, thereby producing an image of decreased quality. This aberration is due to the different variation of index refraction with color from one material to another.
To avoid bending of the light transmitted through the embedded prism plate, the prism and the fluid in which it is immersed are made of materials selected to have matching indices of refraction. As is well known, the index of refraction of a material is proportional to the reciprocal of the velocity of light propagated in the material, and such velocity varies from one material to another. Thus, as the light passes from one material to another with a different index of refraction, the light beam is bent. Accordingly, an embedded prism must be constructed with materials having the same index of refraction insofar as possible, if beam bending is to be minimized.
The embedded prism polarizing beam splitter, e.g. an embedded MacNeille-type polarizing beam splitter, is useful in a wide variety of applications. One example of such an application is a color projection system employing a liquid crystal light valve. Examples of such projection systems are described in U.S. Pat. No. 4,343,535 to Bleha, and in U.S. Pat. No. 4,650,286 to Koda. For example, some systems of this type may use a prism plate of Schott BK-7 glass having an index of refraction of 1.518298 at 554.5 nanometers in conjunction with a Cargille Code 1160 fluid having an index of refraction of 1.515 at the same wavelength at a temperature of 25 degrees Celsius.
In such a color projection system, as described in detail in these patents, light from a light source is reflected from a MacNeille prism to a liquid crystal light valve. This in turn causes the light valve to retroreflect light of a particular polarization in accordance with modulation imposed on the light valve by an image generator, such as a cathode ray tube. The uniquely polarized light modulated and retroreflected from the light valve is then transmitted through the embedded MacNeille prism and projected via a projection lens. The optical fluid minimizes bending of light at the fluid-glass interfaces as compared to a glass-interface. However, optical fluids do have some drawbacks.
In general, there are a number of desired characteristics for optical fluids when used in systems such as the above-described liquid crystal light valve color projection systems. For example, it is desirable that the optical fluid does not have toxic properties, has low vapor pressure, no disagreeable odor, does not have a high flash point, and has a freezing point which is below the operating temperature. Also it is desirable for the fluid to be soluble in commercial solvents for cleanup and be compatible with other optical materials. Conventional optical fluids, such as the Cargille Code 1160, generally meet these requirements.
However, there are additional desirable features which are not entirely met by conventional optical fluids. For example, Cargille Code 1160 is a chemical mixture which results in some undesirable features. First, it appears to contain a certain amount of contaminants and it is difficult to test for purity. Also, it does not appear to be consistent from batch to batch. As a result its properties, such as transmission properties, vary from batch to batch apparently due to variations in manufacturing processes. Furthermore, this fluid is relatively expensive and exhibits poor recovery from freezing. Moreover, because it has a relatively high viscosity, this fluid exhibits thermal instability, which results in refraction variations also known as heat waves. This results in fluctuations in the resulting image that limit resolution and cause oscillations irregularly across the image. For example, Cargille Code 1160 has a viscosity of 53 centistokes at 17 degrees Celsius as measured with a Zahn cup-type viscosimeter. Also, there appears to be some problems with the accumulation of particles on the glass plates in the MacNeille prisms.
Further, the Cargille Code 1160 optical fluid has relatively poor transmission throughout the appropriate visible optical spectrum. This is particularly noticeable in applications where the total optical path length in the fluid is long. In addition, this optical fluid exhibits low transmission in the blue region. As a result, in three color systems, red and green channels must be turned down to achieve a relatively higher level of blue transmission. This lowers the overall brightness of the system.
Thus, it would be desirable to provide an improved optical system which utilizes a superior optical fluid that has improved transmission. It would be particularly desirable for the fluid to have improved transmission in the blue spectrum to improve the color temperature, and thus avoid the necessity of lowering the red and green channels. It would also be desirable to provide a system utilizing an optical fluid which has low viscosity to minimize thermal irregularities. Furthermore, it would be desirable to provide such an optical fluid which is low in cost, which recovers well from freezing and which minimizes the formation of particles on internal glass plates. Also, it would be desirable to provide such a system utilizing a fluid which is pure and free of contaminants and which has consistently reproducible optical properties from batch to batch.